Hitherto, a drain trap for storing water therein is provided at a drain pipe at a washstand, a sink, or the like in order to prevent an offensive odor from entering a room from the sewerage side and to prevent creatures such as insects from entering the room from the sewerage side. However, when a washstand, a sink, or the like is not used for a long period of time due to a reason such as absence from a house, no renter for an apartment, or the like, there is a concern that seal water vaporizes (disappears) and an offensive odor enters a room from the sewerage side or creatures such as insects enter the room from the sewerage side. Thus, in order to prevent vaporization of the seal water, an oil is added to the seal water to cover the surface of the water (e.g., see Non-Patent Literature 1).
However, the specific gravity of the oil is lower than that of water, and thus there is a drawback that when the oil is added, the oil has difficulty in moving in the seal water from the drain outlet side to the sewerage side. In addition, there is also a problem that water in a drain trap is frozen in a cold region, and thus, hitherto, ethylene glycol is used for a cold region in some cases.